Redo Log For Append Only Storage Scheme

ABSTRACT

A new snapshot of a storage volume is created by suppressing write requests. Once pending write requests from the computing nodes are completed, storage nodes create a new snapshot for the storage volume by allocating a new segment to the new snapshot. Data is written in segments and Metadata is stored in the segments indicating LBAs of data stored therein and offsets within the segments at which data for LBAs are stored. For write requests, index entries are stored in an index buffer for a segment and written to the segment when the buffer is full. A redo entry is created in a redo buffer for multiple storage volumes and slices for each write request. Write requests are acknowledged when the redo buffer is written to redo segments on a storage device. On restart, index buffers are reconstructed from the redo segments if needed.

BACKGROUND Field of the Invention

This invention relates to orchestration of roles in an applicationinstantiated in a distributed storage and computation system.

Background of the Invention

In many contexts, it is helpful to be able to return a database to anoriginal state or some intermediate state. In this manner, changes tosoftware or other database configuration parameters may be testedwithout fear of corrupting critical data.

The systems and methods disclosed herein provide an improved approachfor creating snapshots of a database and returning to a previoussnapshot.

BRIEF DESCRIPTION OF THE DRAWINGS

In order that the advantages of the invention will be readilyunderstood, a more particular description of the invention brieflydescribed above will be rendered by reference to specific embodimentsillustrated in the appended drawings. Understanding that these drawingsdepict only typical embodiments of the invention and are not thereforeto be considered limiting of its scope, the invention will be describedand explained with additional specificity and detail through use of theaccompanying drawings, in which:

FIG. 1 is a schematic block diagram of a network environment forimplementing methods in accordance with an embodiment of the presentinvention;

FIG. 2 is a process flow diagram of a method for coordinating snapshotcreation with compute nodes and storage nodes in accordance with anembodiment of the present invention;

FIG. 3 is a schematic diagram illustrating the storage of data within astorage node in accordance with an embodiment of the present invention;

FIG. 4 is a process flow diagram of a method for processing writerequests in a storage node in accordance with an embodiment of thepresent invention;

FIG. 5 is a process flow diagram of a method for processing a snapshotinstruction by a storage node in accordance with an embodiment of thepresent invention;

FIG. 6 is a process flow diagram of a method for performing garbagecollection on segments in accordance with an embodiment of the presentinvention;

FIG. 7 is a process flow diagram of a method for reading data from asnapshot in accordance with an embodiment of the present invention;

FIG. 8 is a process flow diagram of a method for cloning a snapshot inaccordance with an embodiment of the present invention;

FIG. 9 illustrates a snapshot hierarchy created in accordance with anembodiment of the present invention;

FIG. 10 is a process flow diagram of a method for rolling back to aprior snapshot in accordance with an embodiment of the presentinvention;

FIG. 11 illustrates the snapshot hierarchy of FIG. 9 as modifiedaccording to the method of FIG. 10 in accordance with an embodiment ofthe present invention;

FIG. 12 is a process flow diagram of a method for reading from a clonevolume in accordance with an embodiment of the present invention;

FIG. 13 is a diagram illustrating the block processing of write requestsin accordance with an embodiment of the present invention;

FIG. 14 is a process flow diagram of a method for block processing writerequests in accordance with an embodiment of the present invention;

FIG. 15 is a diagram illustrating data structures for implementing aredo log in accordance with an embodiment of the present invention;

FIG. 16 is a process flow diagram of a method for using a redo log inaccordance with an embodiment of the present invention;

FIG. 17 is a process flow diagram of a method for recovering data from aredo log in accordance with an embodiment of the present invention; and

FIG. 18 is a schematic block diagram of an example computing devicesuitable for implementing methods in accordance with embodiments of theinvention.

DETAILED DESCRIPTION

Referring to FIG. 1, the methods disclosed herein may be performed usingthe illustrated network environment 100. The network environment 100includes a storage manager 102 that coordinates the creation ofsnapshots of storage volumes and maintains records of where snapshotsare stored within the network environment 100. In particular, thestorage manager 102 may be connected by way of a network 104 to one ormore storage nodes 106, each storage node having one or more storagedevices 108, e.g. hard disk drives, flash memory, or other persistent ortransitory memory. The network 104 may be a local area network (LAN),wide area network (WAN), or any other type of network including wired,fireless, fiber optic, or any other type of network connections.

One or more compute nodes 110 are also coupled to the network 104 andhost user applications that generate read and write requests withrespect to storage volumes managed by the storage manager 102 and storedwithin the memory devices 108 of the storage nodes 108.

The methods disclosed herein ascribe certain functions to the storagemanager 102, storage nodes 106, and compute node 110. The methodsdisclosed herein are particularly useful for large scale deploymentincluding large amounts of data distributed over many storage nodes 106and accessed by many compute nodes 110. However, the methods disclosedherein may also be implemented using a single computer implementing thefunctions ascribed herein to some or all of the storage manager 102,storage nodes 106, and compute node 110.

Referring to FIG. 2, the illustrated method 200 may be performed inorder to invoke the creation of a new snapshot. Other than a currentsnapshot, which is still subject to change, a snapshot captures thestate of a storage volume at a moment in time and is preferably notaltered in response to subsequent writes to the storage volume.

The method 200 includes receiving, by the storage manager 102 a requestto create a new snapshot for a storage volume. A storage volume asreferred to herein may be a virtual storage volume that may divided intoindividual slices. For example, storage volumes as described herein maybe 1 TB and be divided into 1 GB slices. In general, a slice and itssnapshot are stored on a single storage node 106, whereas a storagevolume may have the slices thereof stored by multiple storage nodes 106.

The request received at step 202 may be received from a human operatoror generated automatically, such as according to backup schedulerexecuting on the storage manager 102 or some other computing device. Thesubsequent steps of the method 200 may be executed in response toreceiving 202 the request

The method 200 may include transmitting 204 a quiesce instruction to allcompute nodes 110 that are associated with the storage volume. Forexample, all compute nodes 110 that have pending write requests to thestorage volume. In some embodiments, the storage manager 102 may store amapping of compute nodes 110 to a particular storage volume used by thecompute nodes 110. Accordingly, step 204 may include sending 204 thequiesce instruction to all of these compute nodes. Alternatively, theinstruction may be transmitted 204 to all compute nodes 110 and includean identifier of the storage volume. The compute nodes 110 may thensuppress any write instructions referencing that storage volume.

The quiesce instruction instructs the compute nodes 110 that receive itto suppress 206 transmitting write requests to the storage nodes 106 forthe storage volume referenced by the quiesce instruction. The quiesceinstruction may further cause the compute nodes 110 that receive it toreport 208 to the storage manager 102 when no write requests are pendingfor that storage volume, i.e. all write requests issued to one or morestorage nodes 106 and referencing slices of that storage volume havebeen acknowledged by the one or more storage nodes 106.

In response to receiving the report of step 208 from one or more computenodes, e.g. all compute nodes that are mapped to the storage node thatis the subject of the snapshot request of step 202, the storage manager102 transmits 210 an instruction to the storage nodes 106 associatedwith the storage volume to create a new snapshot of that storage volume.Step 210 may further include transmitting 210 an instruction to thecompute nodes 110 associated with the storage volume to commence issuingwrite commands to the storage nodes 106 associated with the storagevolume. In some embodiments, the instruction of step 110 may include anidentifier of the new snapshot. Accordingly, subsequent input/outputoperations (IOPs) transmitted 214 from the compute nodes may referencethat snapshot identifier. Likewise, the storage node 106 may associatethe snapshot identifier with data subsequently written to the storagevolume, as described in greater detail below.

In response to receiving 210 the instruction to create a new snapshot,each storage node 106 finalizes 212 segments associated with the currentsnapshot, which may include performing garbage collection, as describedin greater detail below. In addition, subsequent IOPs received by thestorage node may also be processed 216 using the new snapshot as thecurrent snapshot, as is also described in greater detail below.

Referring to FIG. 3, the method by which slices are allocated,reassigned, written to, and read from may be understood with respect tothe illustrated data storage scheme. The data of the storage scheme maybe stored in transitory or persistent memory of the storage node 106,such as in the storage devices 108.

For each logical volume, the storage manager 102 may store and maintaina volume map 300. For each slice in the logical volume, the volume mapmay include an entry including a node identifier 302 identifying thestorage node 106 to which the slice is assigned and an offset 304 withinthe logical volume at which the slice begins. In some embodiments,slices are assigned both to a storage node 106 and a specific storagedevice hosted by the storage node 106. Accordingly, the entry mayfurther include a disk identifier of the storage node 106 referencingthe specific storage device to which the slice is assigned.

The remaining data structures of FIG. 3 are stored on each storage node106. The storage node 106 may store a slice map 308. The slice map 308may include entries including a local slice identifier 310 that uniquelyidentifies each slice of the storage node 106, e.g. each slice of eachstorage device hosted by the storage node 106. The entry may furtherinclude a volume identifier 312 that identifies the logical volume towhich the local slice identifier 310 is assigned. The entry may furtherinclude the offset 304 within the logical volume of the slice of thelogical volume assigned to the storage node 106.

In some embodiments, an entry in the slice map 308 is created for aslice of the logical volume only after a write request is received thatreferences the offset 304 for that slice. This further supports theimplementation of overprovisioning such that slices may be assigned to astorage node 106 in excess of its actual capacity since the slice isonly tied up in the slice map 308 when it is actually used.

The storage node 106 may further store and maintain a segment map 314.The segment map 314 includes entries either including or correspondingto a particular physical segment identifier (PSID) 316. For example, thesegment map 314 may be in an area of memory such that each address inthat area corresponds to one PSID 316 such that the entry does notactually need to include the PSID 316. The entries of the segment map314 may further include a slice identifier 310 that identifies a localslice of the storage node 106 to which the PSID 316 has been assigned.The entry may further include a virtual segment identifier (VSID) 318.As described in greater detail below, each time a segment is assigned tological volume and a slice of a logical volume, it may be assigned aVSID 318 such that the VSIDs 318 increase in value monotonically inorder of assignment. In this manner, the most recent PSID 316 assignedto a logical volume and slice of a logical volume may easily bedetermined by the magnitude of the VSIDs 318 mapped to the PSIDs 316. Insome embodiments, VSIDs 318 are assigned in a monotonically increasingseries for all segments assigned to volume ID 312. In other embodiments,each offset 304 and its corresponding slice ID 310 is assigned VSIDsseparately, such that each slice ID 310 has its own corresponding seriesof monotonically increasing VSIDs 318 assigned to segments allocated tothat slice ID 310.

The entries of the segment map 314 may further include a data offset 320for the PSID 316 of that entry. As described in greater detail below,when data is written to a segment it may be written at a first openposition from a first end of the segment. Accordingly, the data offset320 may indicate the location of this first open position in thesegment. The data offset 320 for a segment may therefore be updated eachtime data is written to the segment to indicate where the new first openposition is.

The entries of the segment map 314 may further include a metadata offset322. As described in detail below, for each write request written to asegment, a metadata entry may be stored in that segment at a first openposition from a second end of the segment opposite the first end.Accordingly, the metadata offset 322 in an entry of the segment map 314may indicate a location of this first open position of the segmentcorresponding to the entry.

Each PSID 316 corresponds to a physical segment 324 on a device hostedby the storage node 106. As shown, data payloads 326 from various writerequests are written to the physical segment 324 starting from a firstend (left) of the physical segment. The physical segment may furtherstore index pages 328 such that index pages are written starting from asecond end (right) of the physical segment 324.

Each index page 328 may include a header 330. The header 330 may becoded data that enables identification of a start of an index page 328.The entries of the index page 328 each correspond to one of the datapayloads 326 and are written in the same order as the data payloads 326.Each entry may include a logical block address (LBA) 332. The LBA 332indicates an offset within the logical volume to which the data payloadcorresponds. The LBA 332 may indicate an offset within a slice of thelogical volume. For example, inasmuch as the PSID 316 is mapped to aslice ID 310 that is mapped to an offset 304 within a particular volumeID 312, maps 308 and 314, and an LBA 332 within the slice may be mappedto the corresponding offset 304 to obtain a fully resolved addresswithin the logical volume.

In some embodiments, the entries of the index page 328 may furtherinclude a physical offset 334 of the data payload 326 corresponding tothat entry. Alternatively or additionally, the entries of the index page328 may include a size 336 of the data payload 326 corresponding to theentry. In this manner, the offset to the start of a data payload 326 foran entry may be obtained by adding up the sizes 336 of previouslywritten entries in the index pages 328.

The metadata offset 322 may point to the last index page 328 (furthestfrom right in illustrated example) and may further point to the firstopen entry in the last index page 328. In this manner, for each writerequest, the metadata entry for that request may be written to the firstopen position in the last index page 328. If all of the index pages 328are full, a new index page 328 may be created and stored at the firstopen position from the second end and the metadata for the write requestmay be added at the first open position in that index page 328.

The storage node 106 may further store and maintain a block map 338. Ablock map 338 may be maintained for each logical volume and/or for eachslice offset of each logical volume, e.g. for each local slice ID 310which is mapped to a slice offset and logical volume by slice map 308.The entries of the block map 338 map include entries corresponding toeach LBA 332 within the logical volume or slice of the logical volume.The entries may include the LBA 332 itself or may be stored at alocation within the block map corresponding to an LBA 332.

The entry for each LBA 332 may include the PSID 316 identifying thephysical segment 324 to which a write request referencing that LBA waslast written. In some embodiments, the entry for each LBA 332 mayfurther indicate the physical offset 334 within that physical segment324 to which the data for that LBA was written. Alternatively, thephysical offset 324 may be obtained from the index pages 328 of thatphysical segment. As data is written to an LBA 332, the entry for thatLBA 332 may be overwritten to indicate the physical segment 324 andphysical offset 334 within that segment 324 to which the most recentdata was written.

In embodiments implementing multiple snapshots for a volume and slice ofa volume, the segment map 314 may additionally include a snapshot ID 340identifying the snapshot to which the PSID 316 has been assigned. Inparticular, each time a segment is allocated to a volume and slice of avolume, the current snapshot identifier for that volume and slice of avolume will be included as the snapshot ID 340 for that PSID 316.

In response to an instruction to create a new snapshot for a volume andslice of a volume, the storage node 106 will store the new currentsnapshot identifier, e.g. increment the previously stored currentsnapshot ID 340, and subsequently allocated segments will include thecurrent snapshot ID 340. PSIDs 316 that are not filled and are allocatedto the previous snapshot ID 340 may no longer be written to. Instead,they may be finalized or subject to garbage collection (see FIGS. 5 and6).

FIG. 4 illustrates a method 400 for executing write instructions by astorage node 106, such as write instructions received from anapplication executing on a compute node 110.

The method 400 includes receiving 402 a write request. The write requestmay include payload data, payload data size, and an LBA as well asfields such as a slice identifier, a volume identifier, and a snapshotidentifier. Where a slice identifier is included, the LBA may be anoffset within the slice, otherwise the LBA may be an address within thestorage volume.

The method 400 may include evaluating 404 whether a PSID 316 isallocated to the snapshot referenced in the write request and whetherthe physical segment 324 corresponding to the PSID 316 (“the currentsegment”) has space for the payload data. In some embodiments, as writerequests are performed with respect to a PSID 316, the amount of datawritten as data 326 and index pages 328 may be tracked, such as by wayof the data offset 320 and metadata offset 322 pointers. Accordingly, ifthe amount of previously-written data 326 and the number of allocatedindex pages 328 plus the size of the payload data and its correspondingmetadata entry exceeds the capacity of the current segment it may bedetermined to be full at step 404.

If the current segment is determined 404 to be full, the method 400 mayinclude allocating 406 a new PSID 316 as the current PSID 316 and itscorresponding physical segment 324 as the current segment for thesnapshot referenced in the write request. In some embodiments, thestatus of PSIDs 316 of the physical storage devices 108 may be flaggedin the segment map 314 as allocated or free as a result of allocationand garbage collection, which is discussed below. Accordingly, a freePSID 316 may be identified in the segment map 314 and flagged asallocated.

The segment map 314 may also be updated 408 to include a slice ID 310and snapshot ID 340 mapping the current PSID 316 to the snapshot ID,volume ID 312, and offset 304 included in the write request. Uponallocation, the current PSID 316 may also be mapped to a VSID (virtualsegment identifier) 318 that will be a number higher than previouslyVSIDs 318 such that the VSIDs increase monotonically, subject, ofcourse, to the size limit of the field used to store the VSID 318.However, the size of the field may be sufficiently large that it is notlimiting in most situations.

The method 400 may include writing 410 the payload data to the currentsegment. As described above, this may include writing 410 payload data326 to the free location closest to the first end of the currentsegment.

The method 400 may further include writing 412 a metadata entry to thecurrent segment. This may include writing the metadata entry (LBA, size)to the first free location closest to the second end of the currentsegment. Alternatively, this may include writing the metadata entry tothe first free location in an index page 328 that has room for it orcreating a new index page 328 located adjacent a previous index page328. Steps 410, 412 may include updating one or more pointers or tablethat indicates an amount of space available in the physical segment,such as a pointer 320 to the first free address closest to the first endand a pointer 322 to the first free address closest to the second end,which may be the first free address before the last index page 328and/or the first free address in the last index page. In particular,these pointers may be maintained as the data offset 320 and metadataoffset in the segment map 314 for the current PSID 316.

The method 400 may further include updating 416 the block map 338 forthe current snapshot. In particular, for each LBA 332 referenced in thewrite request, an entry in the block map 338 for that LBA 332 may beupdated to reference the current PSID 316. A write request may write toa range of LBAs 332. Accordingly, the entry for each LBA 332 in thatrange may be updated to refer to the current PSID 316.

Updating the block map 338 may include evaluating 414 whether an entryfor a given LBA 332 referenced in the write request already exists inthe block map 338. If so, then that entry is overwritten 418 to refer tothe current PSID 316. If not, an entry is updated 416 in the block map318 that maps the LBA 332 to the current PSID 316. In this manner, theblock map 338 only references LBAs 332 that are actually written to,which may be less than all of the LBAs 332 of a storage volume or slice.In other embodiments, the block map 338 is of fixed size and includes anentry for each LBA 332 regardless of whether it has been written topreviously. The block map 338 may also be updated to include thephysical offset 334 within the current segment to which the data 326from the write request was written.

In some embodiments, the storage node 106 may execute multiple writerequests in parallel for the same LBA 332. Accordingly, it is possiblethat a later write can complete first and update the block map 338whereas a previous write request to the same LBA 332 completes later.The data of the previous write request is therefore stale and the blockmap 338 should not be updated.

Suppressing of updating the block map 338 may be achieved by using theVSIDs 318 and physical offset 334. When executing a write request for anLBA, the VSID 318 mapped to the segment 324 and the physical offset 334to which the data is to be, or was, written may be compared to the VSID318 and offset 334 corresponding to the entry in the block map 338 forthe LBA 332. If the VSID 318 mapped in the segment map 314 to the PSID316 in the entry of the block map 338 corresponding to the LBA 332, thenthe block map 338 will not be updated. Likewise, if the VSID 318corresponding to the PSID 316 in the block map 338 is the same as theVSID 318 for the write request and the physical offset 334 in the blockmap 338 is higher than the offset 334 to which the data of the writerequest is to be or was written, the block map 338 will not be updatedfor the write request.

As a result of steps 414-418, the block map 338 only lists the PSID 316where the valid data for a given LBA 332 is stored. Accordingly, onlythe index pages 328 of the physical segment 324 mapped to the PSID 316listed in the block map 338 need be searched to find the data for agiven LBA 332. In instances where the physical offset 334 is stored inthe block map 338, no searching is required.

FIG. 5 illustrates a method 500 executed by a storage node 106 inresponse to the new snapshot instruction of step 210 for a storagevolume. The method 500 may be executed in response to an explicitinstruction to create a new snapshot or in response to a write requestthat includes a new snapshot ID 340. The method 500 may also be executedwith respect to a current snapshot that is still being addressed by newwrite requests. For example, the method 500 may be executed periodicallyor be triggered based on usage.

The method 500 may include allocating 502 a new PSID 316 and itscorresponding physical segment 324 as the current PSID 316 and currentsegment for the storage volume, e.g., by including a slice ID 310corresponding to a volume ID 312 and offset 304 included in the newsnapshot instruction or the write request referencing the new snapshotID 340. Allocating 502 a new segment may include updating 504 an entryin the segment map 314 that maps the current PSID 316 to the snapshot ID340 and a slice ID 310 corresponding to a volume ID 312 and offset 304included in the new snapshot instruction.

As noted above, when a PSID 316 is allocated, the VSID 318 for that PSID316 will be a number higher than all VSIDs 318 previously assigned tothat volume ID 312, and possibly to that slice ID 310 (where slices haveseparate series of VSIDs 318). The snapshot ID 340 of the new snapshotmay be included in the new snapshot instruction or the storage node 106may simply assign a new snapshot ID that is the previous snapshot ID 340plus one.

The method 500 may further include finalizing 506 and performing garbagecollection with respect to PSIDs 316 mapped to one or more previoussnapshots IDs 340 for the volume ID 312 in the segment map 314, e.g.,PSIDs 316 assigned to the snapshot ID 340 that was the current snapshotimmediately before the new snapshot instruction was received.

FIG. 6 illustrates a method 600 for finalizing and performing garbagecollection with respect to segment IDs 340 for a snapshot (“the subjectsnapshot”), which may include the current snapshot or a previoussnapshot. The method 600 may include marking 602 as valid latest-writtendata for an LBA 332 in the PSID 316 having the highest VSID 318 in thesegment map 314 and to which data was written for that LBA 332. Marking602 data as valid may include making an entry in a separate table thatlists the location of valid data or entries for metadata in a givenphysical segment 324 or setting a flag in the metadata entries stored inthe index pages 328 of a physical segment 324, e.g., a flag thatindicates that the data referenced by that metadata is invalid or valid.

Note that the block map 338 records the PSID 316 for the latest versionof the data written to a given LBA 332. Accordingly, any references tothat LBA 332 in the physical segment 324 of a PSID 316 mapped to alower-numbered VSID 318 may be marked 604 as invalid. For the physicalsegment 324 of the PSID 316 in the block map 338 for a given LBA 332,the last metadata entry for that LBA 332 may be found and marked asvalid, i.e. the last entry referencing the LBA 332 in the index page 328that is the last index page 328 including a reference to the LBA 332.Any other references to the LBA 332 in the physical segment 324 may bemarked 604 as invalid. Note that the physical offset 334 for the LBA 332may be included in the block map 334, so all metadata entries notcorresponding to that physical offset 334 may be marked as invalid.

The method 600 may then include processing 606 each segment ID S of thePSIDs 316 mapped to the subject snapshot according to steps 608-620. Insome embodiments, the processing of step 606 may exclude a current PSID316, i.e. the last PSID 302 assigned to the subject snapshot. Asdescribed below, garbage collection may include writing valid data froma segment to a new segment. Accordingly, step 606 may commence with thePSID 316 having the lowest-valued VSID 318 for the subject snapshot. Asany segments 324 are filled according to the garbage collection process,they may also be evaluated to be finalized or subject to garbagecollection as described below.

The method 600 may include evaluating 608 whether garbage collection isneeded for the segment ID S. This may include comparing the amount ofvalid data in the physical segment 324 for the segment ID S to athreshold. For example, if only 40% of the data stored in the physicalsegment 324 for the segment ID S has been marked valid, then garbagecollection may be determined to be necessary. Other thresholds may beused, such as value between 30% and 80%. In other embodiments, theamount of valid data is compared to the size of the physical segment324, e.g., the segment ID S is determined to need garbage collection ifthe amount of valid data is less than X % of the size of the physicalsegment 324, where X is a value between 30 and 80, such as 40.

If garbage collection is determined 608 not to be needed, the method 600may include finalizing 610 the segment ID S. Finalizing may includeflagging the segment ID S in the segment map 314 as full and no longeravailable to be written to. This flag may be stored in another tablethat lists finalized PSIDs 316.

If garbage collection is determined 608 to be needed, then the method600 may include writing 612 the valid data to a new segment. Forexample, if the valid data may be written to a current PSID 316, i.e.the most-recently allocated PSID 316 for the subject snapshot, until itscorresponding physical segment 324 full. If there is no room in thephysical segment 324 for the current PSID 316, step 612 may includeassigning a new PSID 316 as the current PSID 316 for the subjectsnapshot. The valid data, or remaining valid data, may then be writtento the physical segment 324 corresponding to the current PSID 316 forthe subject snapshot.

Note that writing 612 the valid data to the new segment maybe processedin the same manner as for any other write request (see FIG. 4) exceptthat the snapshot ID used will be the snapshot ID 340 of the subjectsnapshot, which may not be the current snapshot ID. In particular, themanner in which the new PSID 316 is allocated to the subject snapshotmay be performed in the same manner described above with respect tosteps 406-48 of FIG. 4. Likewise, the manner in which the valid data iswritten to the current segment may be performed in the same manner asfor steps 410-412 of FIG. 4. In some embodiments, writing of valid datato a new segment as part of garbage collection may also include updatingthe block map with the new location of the data for an LBA 332, such asaccording to steps 414-418 of FIG. 4. When the physical segment 324 ofthe current PSID 316 is found to be full, it may itself be subject tothe process 600 by which it is finalized or subject to garbagecollection.

After the valid data is written to a new segment, the method 600 mayfurther include freeing 614 the PSID Sin the segment map 314, e.g.,marking the entry in segment map 314 corresponding to PSID S as free.

The process of garbage collection may be simplified for PSIDs 316 thatare associated with the subject snapshot in the segment map 314 but arenot listed in the block map 338 with respect to any LBA 332. Thephysical segments 324 of such PSIDs 316 do not store any valid data.Entries for such PSIDs 316 in the segment map 314 may therefore simplybe deleted and marked as free in the segment map 314

FIG. 7 illustrates a method 700 that may be executed by a storage node106 in response to a read request. The read request may be received froman application executing on a compute node 110. The read request mayinclude such information as a snapshot ID, volume ID (and/or slice ID),LBA, and size (e.g. number of 4 KB blocks to read).

The following steps of the method 700 may be initially executed usingthe snapshot ID 340 included in the read request as “the subjectsnapshot,” i.e., the snapshot that is currently being processed tosearch for requested data. The method 700 includes receiving 702 theread request by the storage node 106 and identifying 704 one or morePSIDs 316 in the segment map 314 assigned to the subject snapshot andsearching 706 the metadata entries for these PSIDs 316 for references tothe LBA 332 included in the read request.

The searching of step 706 may be performed in order of decreasing VSID318, i.e. such that the metadata entries for the last allocated PSID 316is searched first. In this manner, if reference to the LBA 332 is found,the metadata of any previously-allocated PSIDs 316 does not need to besearched.

Searching 706 the metadata for a PSID 316 may include searching one ormore index pages 328 of the physical segment 324 corresponding to thePSID 316. As noted above, one or more index pages 328 are stored at thesecond end of the physical segment 324 and entries are added to theindex pages 328 in the order they are received. Accordingly, thelast-written metadata including the LBA 332 in the last index page 328(furthest from the second end of the physical segment 324) in which theLBA 332 is found will correspond to the valid data for that LBA 332. Tolocate the data 326 corresponding to the last-written metadata for theLBA 332 in the physical segment 324, the sizes 336 for allpreviously-written metadata entries may be summed to find a startaddress in the physical segment 324 for the data 326. Alternatively, ifthe physical offset 334 is included, then the data 326 corresponding tothe metadata may be located without summing the sizes 336.

If reference to the LBA 332 is found 708 in the physical segment 324 forany of the PSIDs 316 allocated to the subject snapshot, the data 326corresponding to the last-written metadata entry including that LBA 332in the physical segment 324 mapped to the PSID 316 having the highestVSID 318 of all PSIDs 316 in which the LBA is found will be returned 710to the application that issued the read request.

If the LBA 332 is not found in the metadata entries for any of the PSIDs316 mapped to subject snapshot, the method 700 may include evaluating712 whether the subject snapshot is the earliest snapshot for thestorage volume of the read request on the storage node 106. If so, thenthe data requested is not available to be read and the method 700 mayinclude returning 714 a “data not found” message or otherwise indicatingto the requesting application that the data is not available.

If an earlier snapshot than the subject snapshot is present for thestorage volume on the storage node 106, e.g., there exists at least onePSID 316 mapped to a snapshot ID 340 that is lower than the snapshot ID340 of the subject snapshot ID, then the immediately preceding snapshotID 340 will be set 716 to be the subject snapshot and processing willcontinue at step 704, i.e. the PSIDs 316 mapped to the subject snapshotwill be searched for the LBA 332 in the read request as described above.

The method 700 is particularly suited for reading data from snapshotsother than the current snapshot that is currently being written to. Inthe case of a read request from the current snapshot, the block map 338may map each LBA 332 to the PSID 316 in which the valid data for thatLBA 332 is written. Accordingly, for such embodiments, step 704 mayinclude retrieving the PSID 332 for the LBA 332 in the write requestfrom the block map 338 and only searching 706 the metadata correspondingto that PSID 316. Where the block map 338 stores a physical offset 334,then the data is retrieved from that physical offset within the physicalsegment 314 of the PSID 336 mapped to the LBA 332 of the read request.

In some embodiments, the block map 332 may be generated for a snapshotother than the current snapshot in order to facilitate executing readrequests, such as where a large number of read requests are anticipatedin order to reduce latency. This may include searching the index pages328 of the segments 324 allocated to the subject snapshot and itspreceding snapshots to identify, for each LBA 332 to which data has beenwritten, the PSID 316 having the highest VSID 318 of the PSIDs 316having physical segments 324 storing data written to the each LBA 332.This PSID 316 may then be written to the block map 318 for the each LBA332. Likewise, the physical offset 334 of the last-written data for thatLBA 332 within the physical segment 324 for that PSID 316 may beidentified as described above (e.g., as described above with respect tosteps 704-716).

Referring to FIG. 8, in some instances it may be beneficial to clone astorage volume. This may include capturing a current state of aprincipal copy of a storage volume and making changes to it withoutaffecting the principal copy of the storage volume. For purposes of thisdisclosure a “principal copy” or “principal snapshot” of a storagevolume refers to an actual production copy that is part of a series ofsnapshots that is considered by the user to be the current, official, ormost up-to-date copy of the storage volume. In contrast, a clone volumeis a snapshot created for experimentation or evaluation but changes toit are not intended by the user to become part of the production copy ofthe storage volume. Stated differently, only one snapshot may be aprincipal snapshot with respect to an immediately preceding snapshot,independent of the purpose of the snapshot. Any other snapshots that areimmediate descendants of the immediately preceding snapshot aresnapshots of a clone volume.

The illustrated method 800 may be executed by the storage manager 102and one or more storage nodes 106 in order to implement thisfunctionality. The method 800 may include receiving 802 a cloneinstruction and executing the remaining steps of the method 800 inresponse to the clone instruction. The clone instruction may be receivedby the storage manager 102 from a user or be generated according to ascript or other program executing on the storage manager 102 or a remotecomputing device in communication with the storage manager 102.

The method 800 may include recording 804 a clone branch in a snapshottree. For example, referring to FIG. 9, in some embodiments, for eachsnapshot that is created for a storage volume, the storage manager 102may create a node S1-S5 in a snapshot hierarchy 900. In response to aclone instruction, the storage manager 102 may create a clone volume andbranch to a node A1 representing the clone volume. In the illustratedexample, a clone instruction was received with respect to the snapshotof node S2. This resulted in the creation of a clone volume representedby node A1 that branches from node S2. Note node S3 and its descendantsare also connected to node S2 in the hierarchy.

In some embodiments, the clone instruction may specify which snapshotthe clone volume is of. In other embodiments, the clone instruction maybe inferred to be a snapshot of a current snapshot. In such embodiments,a new principal snapshot may be created and become the current snapshot.The previous snapshot will then be finalized and be subject to garbagecollection as described above. The clone will then branch from theprevious snapshot. In the illustrated example, if node S2 representedthe current snapshot, then a new snapshot represented by node S3 wouldbe created. The snapshot of node S2 would then be finalized and subjectto garbage collection and the snapshot of the clone volume representedby A1 would be created and node A1 would be added to the hierarchy as adescendent of node S2.

In some embodiments, the clone node A1, and possibly its descendants A2to A4 (representing subsequent snapshots of the clone volume), may bedistinguished from the nodes S1 to S5 representing principal snapshots,such as by means of a flag, a classification of the connection betweenthe node A1 and node S2 that is its immediate ancestor, or by storingdata defining node A1 in a separate data structure.

Following creation of a clone volume, other principal snapshots of thestorage volume may be created and added to represented in the hierarchyby one or more nodes S2 to S5. A clone may be created of any of thesesnapshots and represented by additional clone nodes. In the illustratedexample, node B1 represents a snapshot of a clone volume that is a cloneof the snapshot represented by node S4. Subsequent snapshots of theclone volume are represented by nodes B1 to B3.

Referring again to FIG. 8, the creation of a snapshot for a clone volumeon the storage node 106 may be performed in the identical manner as forany other snapshot, such as according to the methods of FIGS. 2 through6. In particular, one or more segments 806 may be allocated to the clonevolume on storage nodes 106 storing slices of the cloned storage volumeand mapped to the clone volume. IOPs referencing the clone volume may beexecuted 808, such as according to the method 400 of FIG. 4.

In some instances, it may be desirable to store snapshots of a clonevolume on a different storage node 106 than the principal snapshots.Accordingly, the method 800 may include allocating 806 segments to theclone volume on the different storage node 106. This may be invoked bysending a new snapshot instruction referencing the clone volume (i.e.,an identifier of the clone volume) to the different storage node 106 andinstructing one or more compute nodes 110 to route IOPs for the clonevolume to the different storage node 106.

The storage node 102 may store in each node of the hierarchy, dataidentifying one or more storage nodes 106 that store data for thesnapshot represented by that node of the hierarchy. For example, eachnode may store or have associated therewith one or more identifiers ofstorage nodes 106 that store a particular snapshot ID for a particularvolume ID. The node may further map one or more slice IDs (e.g., sliceoffsets) of a storage volume to one storage nodes 106 storing data forthat slice ID and the snapshots for that slice ID.

Referring to FIG. 10, one of the benefits of snapshots is the ability tocapture the state of a storage volume such that it can be restored at alater time. FIG. 10 illustrates a method 1000 for rolling back a storagevolume to a previous snapshot, particularly for a storage volume havingone or more clone volumes.

The method 1000 includes receiving 1002, by the storage manager 102, aninstruction to rollback a storage volume to a particular snapshot SN.The method 1000 may then include processing 1004 each snapshot that is arepresented by a descendent node of the node representing snapshot SN inthe snapshot hierarchy, i.e. snapshots SN+1 to SMAX, where SMAX is thelast principal snapshot that is a descendent of snapshot SN (each“descendent snapshot”). For each descendent snapshot, processing 1004may include evaluating 1006 whether the each descendent is an ancestorof a node representing a snapshot of a clone volume. If not, then thestorage manager 102 may instruct all storage nodes 106 storing segmentsmapped to the descendent snapshot to free 1008 these segments, i.e.delete entries from the segment map referencing the descendent snapshotand marking corresponding PSIDs 316 as free in the segment map 314.

If the descendent snapshot is found 1006 to be an ancestor of a snapshotof a clone volume, then step 1008 is not performed and the snapshot andany segments allocated to it are retained.

FIG. 11 illustrates the snapshot hierarchy following execution of themethod 1000 with respect to the snapshot represented by node S3. As isapparent, snapshot S5 has been removed from the hierarchy and anysegments corresponding to these snapshots will have been freed on one ormore storage nodes 106.

However, since node S4 is an ancestor of clone node B1, it is notremoved and segments corresponding to it are not freed on one or morestorage nodes in response to the roll back instruction. Inasmuch as eachsnapshot contains only data written to the storage volume after it wascreated, previous snapshots may be required to recreate the storagevolume. Accordingly, the snapshots of nodes S3 to S1 are needed tocreate the snapshot of the storage volume corresponding to node B1.

Subsequent principal snapshots of the storage volume will be added asdescendants of the node to which the storage volume was rolled back. Inthe illustrated example, a new principal snapshot is represented by nodeS6 that is an immediate descendent of node S3. Node S4 is only presentdue to clone node B1 and therefore may itself be classified as a clonenode in the hierarchy in response to the rollback instruction of step1002.

Note that FIG. 11 is a simple representation of a hierarchy. There couldbe any number of clone volumes, snapshots of clone volumes, clones ofclone volumes and descendent snapshots of any snapshots of any clonevolume represented by nodes of a hierarchy. Accordingly, to roll back toa particular snapshot of a clone, the method 1000 is the same, exceptthat descendants of a snapshot of a clone volume are treated the same asprincipal snapshots and clones of any of these descendants are treatedthe same as a snapshot of a clone volume.

Referring to FIG. 12, the illustrated method 1200 may be used to executea read request with respect to a storage volume that is represented by ahierarchy generated as described above with respect to FIGS. 8 through11. The illustrated method 1200 may also be executed with respect to astorage volume that includes only principal snapshots that aredistributed across multiple storage nodes, i.e., all the segmentscorresponding to snapshots of the same slice of the storage volume arenot located on the same storage node 106. In that case, the hierarchystored on the storage manager 102 stores the location of the segmentsfor each snapshot and therefore enables them to be located.

The method 1200 may be executed by a storage node 106 (“the currentstorage node”) with information retrieved from the storage manager 102as noted below. The method 1200 may include receiving 1202 a readrequest, which may include such information as a snapshot ID, volume ID(and/or slice ID), LBA, and size (e.g. number of 4 KB blocks to read).

Note that the read request may be issued by an application executing ona compute node 110. The compute node 110 may determine which storagenode 106 to transmit the read request using information from the storagemanager 102. For example, the compute node 110 may transmit a request toobtain an identifier for the storage node 102 storing data for aparticular slice and snapshot of a storage volume. The storage managermay then obtain an identifier and/or address for the storage node 106storing that snapshot and slice of the storage volume from thehierarchical representation of the storage volume and return it to therequesting compute node 110. For example, the storage manager 102 mayretrieve this information from the node in the hierarchy representingthe snapshot included in the read request.

In response to the read request, the current storage node performs thealgorithm illustrated by subsequent steps of the method 1200. Inparticular, the method 1200 may include identifying 1204 segmentsassigned to the snapshot ID of the read request in the segment (“thesubject snapshot”).

The method 1200 may include searching 1206 the metadata of the segmentsidentified in step 1204 for the LBA of the read request. If the LBA isfound, the data from the highest numbered segment having the LBA in itsmetadata is returned, i.e. the data that corresponds to the last-writtenmetadata entry including the LBA.

If the LBA is not found in any of the segments mapped to subjectsnapshot, then the method 1200 may include evaluating 1212 whether thesubject snapshot is the earliest snapshot on the current storage node.If not, then steps processing continues at step 1204 with the previoussnapshot set 1214 as the subject snapshot.

Steps 1204-1214 may be performed in the same manner as for steps 704-714of the method 700, including the various modifications and variationsdescribed above with respect to the method 700.

In contrast to the method 700, if the LBA is not found in any of thesegments corresponding to the subject snapshot for any of the snapshotsevaluated, then the method 1200 may include requesting 1216 a location,e.g. storage node identifier, where an earlier snapshot for the volumeID or slice ID is stored. In response to this request, the storagemanager 102 determines an identifier of a storage node 106 storing thesnapshot corresponding to the immediate ancestor of the earliestsnapshot stored on the current storage node in the hierarchy. Thestorage manager 102 may determine an identifier of the storage node 106relating to the immediate-ancestor snapshot and that stores data for aslice ID and volume ID of the read request as recorded for the ancestornearest ancestor node in the hierarchy of the node corresponding to theearliest snapshot stored on the current storage node.

If the current storage node is found 1218 to be the earliest snapshotfor the storage volume ID and/or slice ID of the read request, then thedata the storage manager 102 may report this fact to the storage node,which will then return 1220 a message indicating that the requested LBAis not available for reading, such as in the same manner as step 714 ofthe method 700.

If another storage node stores an earlier snapshot for the volume IDand/or slice ID of the read request, then the read request may betransmitted 1222 to this next storage node by either the current storagenode or the storage manager 102. The processing may then continue atstep 1202 with the next storage node as the current storage node. Theread request transmitted at step 1222 may have a snapshot ID set to thelatest snapshot ID for the storage volume ID and or slice ID of theoriginal read request.

The method 1200 may be performed repeatedly across multiple storagenodes 106 until the earliest snapshot is encountered or the LBA of theread request is located.

FIG. 13 is a diagram illustrating processing of batches of writerequests. On the compute node 110, a data buffer 1300 stores datapayloads 1302 of write requests all addressed to a same storage node106. An index 1304 stores the LBAs 1306 from the write requests and mayalso store the sizes 1308 of the data payloads from each write requests.The data buffer 1300 may store data payloads for a predetermined timeperiod or until the total amount of data in the data buffer 1300 reachesa threshold size.

Once the buffer is full or a time limit is reached, the data payloads1302 are written 1310 in a block write to the storage device 108 of thestorage node 106 addressed by the write requests. The data payloads 1302are then written as data 326 to a physical segment 324. The physicalsegment 324 may be the current segment identified or allocated asdescribed above with respect to the method 400. In particular, the datapayloads 1302 may be written to the current segment starting at the dataoffset 320 for the current segment. Where the block write exceeds thecapacity of the current segment, a portion may be written to the currentsegment and the remainder written to a newly allocated segment.

The index 1304 may also be written to memory 1310 of the storage node106. For example, the LBAs 1306 and sizes 1308 may be written to anindex page buffer 1312, such as in the form of metadata entries 1314.When the index page buffer 1312 is full it is written to the currentsegment in the form of an index page 328. In particular, the contents ofthe index page buffer 1312 may be written as an index page 328 at thefirst free location from the second end of the current segment. Thecontents of the index page buffer 1312 may be combined with a header toform the index page 328. Likewise, the contents of the index page may becombined with the physical offset locations within the current segmentto which the corresponding payload data 1302 were written.

FIG. 14 illustrates a method 1400 for performing block writes that maybe executed by a compute node 110 and a storage node 106. The method1400 may include, by the compute node 110, buffering 1402 data payloadsby the compute node 110 as well as buffering 1404 metadata for the datapayloads, i.e. the LBA and size of each write request. The compute node110 then transmits 1406 a block write to the storage node. The blockwrite may include both the buffered data payloads as well as thebuffered metadata corresponding to the buffered data payloads.

The storage node 106 receives the data payloads and metadata. Thestorage node 106 writes 1408 the data payloads to the current segment.However, the metadata may be buffered 1410 in an index page buffer 1312and not written to an index page 328 of the current segment.

For example, the method 1400 may include evaluating 1412 whether thestorage node 106 has any pending write requests, such as any pendingwrite requests for the storage device 108 storing the current segment.If so, then the metadata is not written. At a time when there is found1412 to be no pending write requests, the index page buffer 1312 maythen be written 1414 to the current segment. In this manner, the latencyof a write command will be the latency of the block write of the datapayload rather than the latency of two writes (payload and metadata). Insome embodiments, if the index page buffer 1312 is filled to the size ofan index page 328, it will be written regardless of whether there is apending write request. In other embodiments, the index page buffer 1312is large enough that filling of the index page buffer is not likely tooccur. In such embodiments, the contents of the index page buffer 1312may be written to multiple index pages 328 which may span multiplephysical segments 324.

Referring to FIG. 15, in the storage scheme described above, each writerequest requires two write operations to the storage device 108, a firstwrite with the payload data and a second write with metadata written toan index page 328. This increases the latency of executing writerequests. Performing block writes of payload and metadata as describedwith respect to FIGS. 13 and 14 helps reduce some of this latency. Theapproach of FIG. 15 illustrates an approach for reducing the latency ofthe two writes to the storage device 108.

In the illustrated approach, a write request 1502 includes payload data1504 and metadata 1506. The metadata 1506 may include a volumeidentifier, slice identifier (e.g., slice offset), LBA within the slicecorresponding to the slice identifier, or other data. Although thelogical storage unit of a slice of a storage volume is discussed herein,any other definition for a logical storage unit may be used and storedin a like manner.

The metadata 1506 may further include data describing an encryptionalgorithm used to encrypt the payload data, a compression algorithm usedto compress the payload data, a type of the write request according to astorage scheme, parity check data, error correction data, or the like.For example, there may be various types of data entries in an index,such as a data entry that represents written data that is to be retainedand discard entries that are executed as part of an SSD trim command ora LINUX “block discard” command.

The payload data is written to a data buffer 1508 in the memory 1500 ofthe storage node 106. The metadata 1506 is used to create an entry in anindex buffer 1510 (“index entry”) and an entry in a redo buffer 1412(“redo entry”).

The index entry may include some or all of the same data included inentries of the index pages 328, as described above, including LBA 332,physical offset 334, and possibly a size 336. As described below, theindex entry may be generated and written to the index buffer 1510 beforethe payload data is written to a physical segment 324. However, thephysical offset 334 at which the payload data will be written may bedetermined, as described above with respect to FIG. 4, and added to theindex entry. The index entry may include some or all of the other itemsof metadata 1506, such as some or all of the encryption algorithm,compression algorithm, parity check data, type, or the like.

Note that the index buffer 1510 may be for a particular segment 324.That is to say that each index buffer 1510 accumulates data that will bewritten to the index pages of a specific segment 324. Accordingly, eachindex buffer 1510 may be associated with a particular PSID 316 for aparticular physical segment 324. The manner in which data is added tothe index buffer 1510 and formatted may be identical to the manner datais added and formatted in the index pages 328 as described above.Alternatively, the index entries may be modified when written to theindex pages 328 of a physical segment 324 in order to have the order andformatting as described above with respect to FIGS. 3 and 4.

The redo entry for a write request may include the index entry for thewrite request and may further include information describing where theindex entry will be written. In some embodiments, the redo buffer 1512is not specific to a particular segment 324, storage volume or sliceoffset within a storage volume. For example, the redo buffer 1512 mayaccumulate redo entries for all write requests for a particular storagedevice 108 of a storage node 106. As a result, redo entries fordifferent segments 324, different storage volumes, and different slicesof storage volumes may be interleaved within the redo buffer 1512.

Accordingly, redo entries may be labeled with data enabling the redoentry to be mapped to a particular storage volume and slice offsetwithin a storage volume. For example, each redo entry may include a PSID316 of the physical segment 324 to which the index entry of that redoentry will be written. The segment map 314 maps a PSID 316 to acorresponding VSID 318 and slice ID 310 mapped to a volume ID 312 andoffset 304 in the slice map (see discussion of FIG. 3, above).Accordingly, the VSID 318 mapped to the physical segment 324 may beincluded in the redo entry instead. In the following disclosure, thePSID is referenced with the understanding that the VSID could also beused using the mapping provided by the segment map 314.

A write request is acknowledged as complete only after the data buffer1508 has been written to the payload data 1514 of the segment 324 andthe redo buffer 1512 has been written to a redo segment 1518. Note thatthis still requires two writes to be performed. However, latency may bereduced by issuing the data write and the redo write to the devicesimultaneously. In prior approaches these writes are serialized. Withboth writes being performed in parallel and the data write typicallybeing much larger than the metadata write, the total latency will onlybe the latency of a single write command.

When the index buffer 1510 for a segment 324 is full, the index buffer1510 is written to index storage 1516 reserved for the index buffer 1510in the segment 324. The content and format of the index entries aswritten to the index storage 1516 may be as described above with respectto FIGS. 3 and 4. By writing the redo buffer 1512 to the storage device108, the index buffer 1510 need not be written to the storage device 108before acknowledging each write request for which entries are added tothe index buffer 1510, thereby reducing latency.

Referring to FIG. 16, write requests may be processed by the storagenode 106 according to the method 1600. The method 1600 includesreceiving 1602 a write request 1502 and writing 1604 the payload data1504 of the write request 1502 to the data buffer 1508. Note that thewrite request of step 1602 may be a block write request as describedabove with respect to FIGS. 13 and 14.

The method 1600 may further include generating 1606 an index entry andwriting 1608 the index entry to the index buffer 1510. In particular,this may include determining the segment 324 and physical offset withinthe physical segment 324 to which the payload data of the write requestis to be written, as described above with respect to FIGS. 3 and 4. Asalso noted above, the index entry may include some or all of themetadata 1506 included in the write request 1502.

The method 1600 may further include writing 1610 a redo entry to theredo buffer 1512. The content of the redo entry may be as describedabove with respect to FIG. 15. In particular, the redo entry may includethe index entry of step 1606 and additionally include an identifier(PSID) of the segment 324 to which the payload data 1504 of the writerequest will be written.

The method 1600 may further include writing 1612 the contents of thedata buffer to a segment 324, i.e., the segment identified at step 1606.The manner in which this segment 324 is selected and the payload data iswritten may be as described above with respect to FIGS. 3 and 4. Writing1612 the contents may include performing a block write as describedabove with respect to FIGS. 13 and 14. Note that the write of step 1612may or may not include writing of the entries in an index page 328corresponding to the payload data, as will be discussed below.

The method 1600 may further include writing 1614 the contents of theredo buffer 1512 to a redo segment 1518. Steps 1612 and 1614 may beperformed in parallel as shown in FIG. 16. The redo buffer 1512 may besmaller in size than the redo segment 1518. Alternatively, the redobuffer 1512 may be the same size as the redo segment 1518. In any case,the redo buffer 1512 may or may not be full when step 1614 is executed.Accordingly, step 1614 may include writing the contents of the redobuffer 1512 to a redo segment 1518. If no redo segment is available, thestorage node 106 allocates a redo segment 1518. If the contents of theredo segment buffer 1512 are greater than available space in a redosegment 1518, another redo segment 1518 may be allocated and thecontents may be split among the two redo segments 1518.

As is apparent from the above description, there may be multiple redosegments 1518. In order to permit parallel processing, there may bemultiple redo segments 1518 with space available for writing redoentries. Likewise, the write of step 1612 and the write of step 1614 maybe performed in parallel, e.g. added to the queue of a storage device108 for execution rather than waiting for one of the writes 1612, 1614to complete before adding the other write 1612, 1614 to the queue.

Only after the writes of both step 1612 and 1614 are completedsuccessfully does the method 1600 include acknowledging 1616 that thewrite request has completed successfully, such as by transmitting anacknowledgment message to an application on a compute node 110 thatgenerated the write request.

If the index buffer 1618 for the segment 324 written to at step 1612 isdetermined 1618 to be full as a result of the write of step 1608, thenthe index buffer is written 1620 to that segment 324 and that segment324 is finalized 1622, e.g. a flag or other value is set to indicatethat the segment 324 is finalized and will not be added to.

The method 1600 may further include evaluating 1624 for each redosegment 1518 written to at step 1614 whether finalizing of the segmentat step 1622 means that the each redo segment 1518 stores only redoentries for segments 324 that are finalized. As noted above, each redoentry includes a reference to the PSID 324 of the segment to which itcorresponds. If all the PSIDs referenced in the redo segment 1518 aredetermined to correspond to finalized segments 324, then the redosegment 1518 is freed 1626, i.e. marked as free or otherwise madeavailable to be overwritten.

FIG. 17 illustrates a method 1700 for using one or more redo segments1518 to recover from a crash of the storage node 106. The method 1700may include detecting 1702 restarting of the storage node 106. Inresponse, the remaining steps of the method 1700 may be executed.

The method 1700 may include evaluating 1704 whether any redo segments1518 are stored in the storage device 108, whether any redo segments1518 have been allocated but not subsequently freed at step 1626. Ifnot, no further action is required, since there are no redo segments1518 including redo entries for non-finalized segments 324.

If so, then the method 1700 may include reading 1706 the redo segments1518 and identifying the PSIDs referenced. For each PSID referenced, theindex entries included in the redo entries referencing that PSID areretrieved and written 1708 to an index buffer 1510. As described abovewith respect to FIGS. 3 and 4, the index pages 328 and the entriesthereof may be written in order such that it is possible to identify theindex entry corresponding to the last received-write request.

Accordingly, the ordering may be determined from the redo segments 1518.For example, where redo entries may be added to redo segments 1518 inthe order received and redo segments 1518 may be assigned indexes in theorder of allocation. Accordingly, the higher the redo segment index andthe closer to the end of the redo segment 1418, the later the indexentry for that redo entry was created. Accordingly, the order ofcreation of the index entries may be determined from the redo entriesand the index entries may be added to the index buffer 1510 according tothe order of creation.

If the index buffer 1510 for a PSID is full, the index buffer is written1710 to the index storage 1516 of the segment 324 corresponding to thatPSID and that segment 324 is finalized. If all the PSIDs referenced in aredo segment 1518 are finalized, then that redo segment 1518 is freed1710, as described above with respect to FIG. 16. If an index buffer1510 is not full, then its corresponding segment 324 is not finalizedand the redo segment referencing the PSID of the segment 324 is notfreed 1710.

After restarting, processing may continue for new write requests asdescribed above with respect to FIG. 16.

FIG. 18 is a block diagram illustrating an example computing device1800. Computing device 1800 may be used to perform various procedures,such as those discussed herein. The storage manager 102, storage nodes106, compute nodes 110, and hybrid nodes, or any computing devicereferenced herein may have some or all of the attributes of thecomputing device 1800.

Computing device 1800 includes one or more processor(s) 1802, one ormore memory device(s) 1804, one or more interface(s) 1806, one or moremass storage device(s) 1808, one or more Input/output (I/O) device(s)1810, and a display device 1830 all of which are coupled to a bus 1812.Processor(s) 1802 include one or more processors or controllers thatexecute instructions stored in memory device(s) 1804 and/or mass storagedevice(s) 1808. Processor(s) 1802 may also include various types ofcomputer-readable media, such as cache memory.

Memory device(s) 1804 include various computer-readable media, such asvolatile memory (e.g., random access memory (RAM) 1814) and/ornonvolatile memory (e.g., read-only memory (ROM) 1816). Memory device(s)1804 may also include rewritable ROM, such as Flash memory.

Mass storage device(s) 1808 include various computer readable media,such as magnetic tapes, magnetic disks, optical disks, solid-statememory (e.g., Flash memory), and so forth. As shown in FIG. 18, aparticular mass storage device is a hard disk drive 1824. Various drivesmay also be included in mass storage device(s) 1808 to enable readingfrom and/or writing to the various computer readable media. Mass storagedevice(s) 1808 include removable media 1826 and/or non-removable media.

I/O device(s) 1810 include various devices that allow data and/or otherinformation to be input to or retrieved from computing device 1800.Example I/O device(s) 1810 include cursor control devices, keyboards,keypads, microphones, monitors or other display devices, speakers,printers, network interface cards, modems, lenses, CCDs or other imagecapture devices, and the like.

Display device 1830 includes any type of device capable of displayinginformation to one or more users of computing device 1800. Examples ofdisplay device 1830 include a monitor, display terminal, videoprojection device, and the like.

Interface(s) 1806 include various interfaces that allow computing device1800 to interact with other systems, devices, or computing environments.Example interface(s) 1806 include any number of different networkinterfaces 1820, such as interfaces to local area networks (LANs), widearea networks (WANs), wireless networks, and the Internet. Otherinterface(s) include user interface 1818 and peripheral device interface1822. The interface(s) 1806 may also include one or more peripheralinterfaces such as interfaces for printers, pointing devices (mice,track pad, etc.), keyboards, and the like.

Bus 1812 allows processor(s) 1802, memory device(s) 1804, interface(s)1806, mass storage device(s) 1808, I/O device(s) 1810, and displaydevice 1830 to communicate with one another, as well as other devices orcomponents coupled to bus 1812. Bus 1812 represents one or more ofseveral types of bus structures, such as a system bus, PCI bus, IEEE1394 bus, USB bus, and so forth.

For purposes of illustration, programs and other executable programcomponents are shown herein as discrete blocks, although it isunderstood that such programs and components may reside at various timesin different storage components of computing device 1800, and areexecuted by processor(s) 1802. Alternatively, the systems and proceduresdescribed herein can be implemented in hardware, or a combination ofhardware, software, and/or firmware. For example, one or moreapplication specific integrated circuits (ASICs) can be programmed tocarry out one or more of the systems and procedures described herein.

In the above disclosure, reference has been made to the accompanyingdrawings, which form a part hereof, and in which is shown by way ofillustration specific implementations in which the disclosure may bepracticed. It is understood that other implementations may be utilizedand structural changes may be made without departing from the scope ofthe present disclosure. References in the specification to “oneembodiment,” “an embodiment,” “an example embodiment,” etc., indicatethat the embodiment described may include a particular feature,structure, or characteristic, but every embodiment may not necessarilyinclude the particular feature, structure, or characteristic. Moreover,such phrases are not necessarily referring to the same embodiment.Further, when a particular feature, structure, or characteristic isdescribed in connection with an embodiment, it is submitted that it iswithin the knowledge of one skilled in the art to affect such feature,structure, or characteristic in connection with other embodimentswhether or not explicitly described.

Implementations of the systems, devices, and methods disclosed hereinmay comprise or utilize a special purpose or general-purpose computerincluding computer hardware, such as, for example, one or moreprocessors and system memory, as discussed herein. Implementationswithin the scope of the present disclosure may also include physical andother computer-readable media for carrying or storingcomputer-executable instructions and/or data structures. Suchcomputer-readable media can be any available media that can be accessedby a general purpose or special purpose computer system.Computer-readable media that store computer-executable instructions arecomputer storage media (devices). Computer-readable media that carrycomputer-executable instructions are transmission media. Thus, by way ofexample, and not limitation, implementations of the disclosure cancomprise at least two distinctly different kinds of computer-readablemedia: computer storage media (devices) and transmission media.

Computer storage media (devices) includes RAM, ROM, EEPROM, CD-ROM,solid state drives (“SSDs”) (e.g., based on RAM), Flash memory,phase-change memory (“PCM”), other types of memory, other optical diskstorage, magnetic disk storage or other magnetic storage devices, or anyother medium which can be used to store desired program code means inthe form of computer-executable instructions or data structures andwhich can be accessed by a general purpose or special purpose computer.

An implementation of the devices, systems, and methods disclosed hereinmay communicate over a computer network. A “network” is defined as oneor more data links that enable the transport of electronic data betweencomputer systems and/or modules and/or other electronic devices. Wheninformation is transferred or provided over a network or anothercommunications connection (either hardwired, wireless, or a combinationof hardwired or wireless) to a computer, the computer properly views theconnection as a transmission medium. Transmissions media can include anetwork and/or data links, which can be used to carry desired programcode means in the form of computer-executable instructions or datastructures and which can be accessed by a general purpose or specialpurpose computer. Combinations of the above should also be includedwithin the scope of computer-readable media.

Computer-executable instructions comprise, for example, instructions anddata which, when executed at a processor, cause a general purposecomputer, special purpose computer, or special purpose processing deviceto perform a certain function or group of functions. The computerexecutable instructions may be, for example, binaries, intermediateformat instructions such as assembly language, or even source code.Although the subject matter has been described in language specific tostructural features and/or methodological acts, it is to be understoodthat the subject matter defined in the appended claims is notnecessarily limited to the described features or acts described above.Rather, the described features and acts are disclosed as example formsof implementing the claims.

Those skilled in the art will appreciate that the disclosure may bepracticed in network computing environments with many types of computersystem configurations, including, an in-dash vehicle computer, personalcomputers, desktop computers, laptop computers, message processors,hand-held devices, multi-processor systems, microprocessor-based orprogrammable consumer electronics, network PCs, minicomputers, mainframecomputers, mobile telephones, PDAs, tablets, pagers, routers, switches,various storage devices, and the like. The disclosure may also bepracticed in distributed system environments where local and remotecomputer systems, which are linked (either by hardwired data links,wireless data links, or by a combination of hardwired and wireless datalinks) through a network, both perform tasks. In a distributed systemenvironment, program modules may be located in both local and remotememory storage devices.

Further, where appropriate, functions described herein can be performedin one or more of: hardware, software, firmware, digital components, oranalog components. For example, one or more application specificintegrated circuits (ASICs) can be programmed to carry out one or moreof the systems and procedures described herein. Certain terms are usedthroughout the description and claims to refer to particular systemcomponents. As one skilled in the art will appreciate, components may bereferred to by different names. This document does not intend todistinguish between components that differ in name, but not function.

It should be noted that the sensor embodiments discussed above maycomprise computer hardware, software, firmware, or any combinationthereof to perform at least a portion of their functions. For example, asensor may include computer code configured to be executed in one ormore processors, and may include hardware logic/electrical circuitrycontrolled by the computer code. These example devices are providedherein purposes of illustration, and are not intended to be limiting.Embodiments of the present disclosure may be implemented in furthertypes of devices, as would be known to persons skilled in the relevantart(s).

At least some embodiments of the disclosure have been directed tocomputer program products comprising such logic (e.g., in the form ofsoftware) stored on any computer useable medium. Such software, whenexecuted in one or more data processing devices, causes a device tooperate as described herein.

While various embodiments of the present disclosure have been describedabove, it should be understood that they have been presented by way ofexample only, and not limitation. It will be apparent to persons skilledin the relevant art that various changes in form and detail can be madetherein without departing from the spirit and scope of the disclosure.Thus, the breadth and scope of the present disclosure should not belimited by any of the above-described exemplary embodiments, but shouldbe defined only in accordance with the following claims and theirequivalents. The foregoing description has been presented for thepurposes of illustration and description. It is not intended to beexhaustive or to limit the disclosure to the precise form disclosed.Many modifications and variations are possible in light of the aboveteaching. Further, it should be noted that any or all of theaforementioned alternate implementations may be used in any combinationdesired to form additional hybrid implementations of the disclosure.

1. A method comprising: receiving, by a storage node including a storagedevice and a memory device, a plurality of write requests, each writerequest including payload data and metadata referencing a logicalstorage unit, the logical storage unit being one of a plurality oflogical storage units hosted by the storage node; and for each writerequest, performing, by the storage node: writing the payload data ofthe each write request to an offset within a segment assigned to thelogical storage unit of the each write request on the storage device;writing an index entry to an index buffer in the memory device, theindex buffer unique to the segment assigned to the logical storage unit,the index entry including the metadata of the each write request and theoffset; and writing a redo entry to a redo buffer in the memory device,the redo buffer being common to the plurality of logical storage units,the redo entry including the metadata of the each write request, theoffset, and an identifier of the segment assigned to the logical storageunit; and acknowledging the write request to a source of the writerequest only after the redo buffer is written to redo storage on thestorage device.
 2. The method of claim 1, wherein writing the redoentries for the plurality of write requests comprises writing the redoentries to the redo buffer in an order processed without segregationaccording to the logical storage units of the plurality of writerequests.
 3. The method of claim 1, further comprising, for each logicalstorage unit of the plurality of logical storage units: when the indexbuffer unique to the segment assigned to the each logical storage unitis full, writing, by the storage node, the index buffer to an indexstorage portion of the segment assigned to the each logical storageunit.
 4. The method of claim 3, further comprising, for each logicalstorage unit of the plurality of logical storage units: after writingthe index buffer to the index storage portion of the segment assigned tothe logical storage unit, flagging, by the storage node, the segmentassigned to the each logical storage unit as being finalized.
 5. Themethod of claim 4, wherein writing the redo buffer to the storage devicecomprises: writing the redo buffer to one of a plurality of redosegments in the storage device.
 6. The method of claim 5, furthercomprising, for each redo segment of the plurality of redo segments:when, for each and every redo entry in the each redo segment, a segmentreferenced by the segment identifier in the each redo entry is flaggedas finalized, freeing the each redo segment to be overwritten.
 7. Themethod of claim 7, further comprising, in response to detectingrestarting of the storage node, for each segment identifier referencedin the redo entries of the plurality of redo segments: reconstructing anindex buffer for the each segment identifier from the redo entries ofthe plurality of redo entries referencing the each segment identifier;and writing the index buffer to the index storage portion of a segmentreferenced by the each segment identifier.
 8. The method of claim 1,wherein the metadata of each write request of the plurality of writerequests includes a logical block address within the logical storageunit referenced by the metadata of the each write request.
 9. The methodof claim 1, wherein each logical storage unit of the plurality oflogical storage units is a slice of a logical storage volume.
 10. Themethod of claim 1, further comprising writing the redo buffer to thestorage device and writing the payload of the each write request to thestorage device in parallel.
 11. A system comprising one or moreprocessing devices, one or more processing devices operably coupled tothe one or more processing devices, and one or more storage devicesoperably coupled to the one or more processing devices, the one or morememory devices storing executable code effective to cause the one ormore processing devices to: receive a plurality of write requests, eachwrite request including payload data and metadata referencing a logicalstorage unit, the logical storage unit being one of a plurality oflogical storage units hosted by the storage node; and for each writerequest: write the payload data of the each write request to an offsetwithin a segment assigned to the logical storage unit of the each writerequest on the one or more storage devices; write an index entry to anindex buffer in the one or more memory devices, the index buffer uniqueto the segment assigned to the logical storage unit, the index entryincluding the metadata of the each write request and the offset; andwrite a redo entry to a redo buffer in the one or more memory devices,the redo buffer being common to the plurality of logical storage units,the redo entry including the metadata of the each write request, theoffset, and an identifier of the segment assigned to the logical storageunit; and acknowledge the write request to a source of the write requestonly after the redo buffer is written to redo storage on the one or morestorage devices.
 12. The system of claim 11, wherein the executable codeis further effective to cause the one or more processing devices towrite the redo entries for the plurality of write requests by writingthe redo entries to the redo buffer in an order processed withoutsegregation according to the logical storage units of the plurality ofwrite requests.
 13. The system of claim 11, wherein the executable codeis further effective to cause the one or more processing devices to, foreach logical storage unit of the plurality of logical storage units:when the index buffer unique to the segment assigned to the each logicalstorage unit is full, writing the index buffer to an index storageportion of the segment assigned to the each logical storage unit. 14.The system of claim 13, wherein the executable code is further effectiveto cause the one or more processing devices to, for each logical storageunit of the plurality of logical storage units: after writing the indexbuffer to the index storage portion of the segment assigned to thelogical storage unit, flag the segment assigned to the each logicalstorage unit as being finalized.
 15. The system of claim 14, wherein theexecutable code is further effective to cause the one or more processingdevices to write the redo buffer to the one or more storage devices by:writing the redo buffer to one of a plurality of redo segments in theone or more storage devices.
 16. The system of claim 15, wherein theexecutable code is further effective to cause the one or more processingdevices to, for each redo segment of the plurality of redo segments:when, for each and every redo entry in the each redo segment, a segmentreferenced by the segment identifier in the each redo entry is flaggedas finalized, free the each redo segment to be overwritten.
 17. Thesystem of claim 17, wherein the executable code is further effective tocause the one or more processing devices to, in response to detectingrestarting of the storage node, for each segment identifier referencedin the redo entries of the plurality of redo segments: reconstruct anindex buffer for the each segment identifier from the rodeo entries ofthe plurality of redo entries referencing the each segment identifier;and write the index buffer to the index storage portion of a segmentreferenced by the each segment identifier.
 18. The system of claim 11,wherein the metadata of each write request of the plurality of writerequests includes a logical block address within the logical storageunit referenced by the metadata of the each write request.
 19. Thesystem of claim 11, wherein each logical storage unit of the pluralityof logical storage units is a slice of a logical storage volume.
 20. Thesystem of claim 11, wherein the executable code is further effective tocause the one or more processing devices to write the redo buffer to theone or more storage devices and write the payload of the each writerequest to the one or more storage devices in parallel.